Pumps can be used in wells to produce production fluids to the surface. One well known type of pump is a hydraulically actuated pump known as the PowerLift I, such as disclosed in U.S. Pat. Nos. 2,943,576; 4,118,154; and 4,214,854. Details of a system having this type of pump are reproduced in FIG. 1. The pump 30 deploys downhole in tubing 16 disposed in a wellbore casing 12. Surface equipment 20 injects power fluid (e.g., produced water or oil) down the tubing 16 to the pump 30. The power fluid enters the pump's inlet 32 and operates the pump 30 internally between upstrokes and downstrokes. In its upstroke, the pump 30 draws production fluid from below a packer 14 into the pump's intake 34. As shown, the production fluid may enter the wellbore's casing 12 through perforations 13. Subsequently operated in its downstroke, the pump 30 discharges the produced fluid and spent power fluid into the tubing 16 via ports 36. The discharged fluid then passes through ports 18 in the production tubing 16 and eventually travels via the tubing-casing annulus to the surface equipment 20 for handling.
Internal details of the pump 30 and its operation are shown in FIGS. 2A-2B. The pump 30 has an engine piston 50, a reversing valve 60, and a pump piston 70. A rod 55 interconnects the engine piston 50 to the pump piston 70 so that the two pistons 50/70 move together in the pump 30. Power fluid used to actuate the pump 30 enters the pump 30 via inlet 32 and travels into an engine barrel 40 via ports 42. Inside the barrel 40, the power fluid acts on the engine piston 50. The reversing valve 60 within the engine piston 50 alternately directs the power fluid above and below the piston 50, causing the piston 50 to reciprocate within the engine's barrel 40. In the upstroke shown in FIG. 2A, mechanical force from a push rod 62 initiates the shifting of the reversing valve 60 downward, after which hydraulic force from the fluid continues to shift the valve 60 downward. This shifting diverts the power fluid to the volume of the barrel 40 above the engine piston 50, and the buildup of power fluid causes the engine piston 50 to move downward in the engine's barrel 40. In the downstroke shown in FIG. 2B, mechanical force and then hydraulic force shift the reversing valve 60 upward. The power fluid fills the barrel's volume below the engine piston 50 and causes the piston 50 to move upward.
The pump piston 70 connected to the engine piston 50 by rod 55 moves in tandem with the engine piston 50. When moved, the pump piston 70 operates similar to a conventional sucker rod pump. At the start of the upstroke shown in FIG. 2A, a traveling valve 75 closes, and a standing valve 35 opens. The fluid in the piston barrel 45 above the pump piston 70 is then displaced out of the pump's barrel 45 via port 36 as the pump piston 70 continues the upstroke. The fluid passes out tubing port 18 and then to the surface.
The upstroke reduces the pressure in the barrel 45 below the pump piston 70 so that the resulting suction allows production fluid to enter the barrel 45 through the open standing valve 34. At the start of the downstroke shown in FIG. 2B, the traveling valve 75 opens, and the standing valve 34 closes. This permits the production fluid that entered the lower part of the barrel 45 below the pump piston 70 to move above the piston 70 through the open traveling valve 75. In this way, this moved production fluid can be discharged to the surface on the next upstroke.
The hydraulically actuated pump 30 is preferred in many installations because initial movement of the reversing valve 60 is mechanically actuated. This allows the pump 30 to operate at low speeds and virtually eliminates the chances that the pump 30 will stall during operation. Unfortunately, the pump 30 can suffer from problems with gas lock, especially in a wellbore that produces excessive compressible fluids, such as natural gas, along with incompressible liquids, such as oil and water.
During operation, for example, the pump 30 can easily draw gas through the standing valve 34 during the piston's upstroke. On the downstroke with the standing valve 34 closed, incompressible fluid in the lower volume of the piston barrel 45 is expected to force the traveling valve 75 open. Because gas between the traveling valve 75 and the standing valve 34 will compress, the hydrostatic head of the fluid above the traveling valve 75 may keep the traveling valve 75 from opening. On the upstroke, the gas and liquid above the standing valve 34 may then prevent any more fluid from being drawn into the pump barrel 45 because the compressed gas merely expands to fill the expanding volume. When this occurs, the pump 30 will alternatingly cycle through upstrokes and downstrokes, but it will simply compress and expand the gas in the pump barrel 45 caught between the standing valve 34 and the traveling valve 75. When this gas lock occurs, the pump 30 fails to move any liquid to the surface.
Because gas lock can be an issue, operators may use other types of pumps that minimize the possibility of gas lock. One such pump is the Type F pump such as disclosed in U.S. Pat. No. Re 24,812. Functionally, the Type F pump operates in a similar way to the PowerLift I pump described above. To minimize gas lock, the Type F pump pressurizes produced fluid to discharge pressure. However, the Type F pump is entirely hydraulically shifted without the mechanical initiation found in the PowerLift I type pump so that the Type F pump can stall when operated at slow speeds. In addition, the Type F pump uses a bleed valve at the pump's discharge, which can be undesirable in some implementations.
What is needed is a hydraulically actuated pump that can operate at slow speeds but that can also reduce or prevent issues with gas lock conventionally found in such pumps.